A friend had an '03 4X4, short bed, crew cab, 2500. Has Gibson exhaust and Hypertech programmer.
I was with this person when they put 21 gallons in. Trip odo was reset at prior fill up. Showed 425 miles. Most of this was their commute, 80 miles - round trip. Mostly 2-lane 55 mph roads. Some city mixed in.
I personnally drove it pulling their 21' Malibu tourny boat. 4 adults, full fuel in both, ice chests, luggage. I got 7.8 driving it 75-80 to Barstow. The owner said it gets a whole lot better at 70 and below. They drove 2nd half (Barstow-Havasu) at 65-70 and got 8.8.